I'm no expert on this stuff so am a little hesitant to 'chip' in but what the hell.
I've tuned a few cars a little and even had an old chevy I fixed up years ago but I made more mistakes than good decisions so don't trust me for anything.
Now, it seems like to me after spending some hard earned money and seeing my friends spend their hard earned money on American cars and imports that there aren't any cheap magic bullets that suddenly make your car faster. I'm not sure if the OP was asking about that kind of thing…. But...
Cars, like a lot of things, are systems. You can add one thing and it might actually decrease performance, add a second or third item to exploit that first thing and suddenly it becomes useful. I remember I had a Dodge Daytona (piece of **** really) that I screwed around with the Blow Off valve and ran a vacuum hose to the interior of the car. I could turn a dial and get that boost cranked up so high the engine would start pinging and cutting out under full throttle. I could've added bigger injectors and a better fuel pump but it was a Dodge Daytona fer Pete's sake. Torque steer alone would keep it from being a 'fast' car. The vacuum hose made a cool sound inside the car though.
With a vehicle like the 4Runner there doesn't seem to be a lot of 'bolt-on' performance mods available that will upgrade the 'system' for little money (relatively speaking, of course).
I've built up old VW engines and we could actually get em going pretty good within their limits. Doing wheelies through stoplights was fun at the time. I've got a friend who has a shop specializing in German tuning (BMWs, VW, Audis etc) and they can do some amazing stuff - but it ain't cheap and it ain't 'bolt on'.
When I used to be into Datsuns the best upgrade was to pull the engine an pop in a 280ZX turbo engine (or drop in a V8). Below that was going from a 2carb to a 3carb setup. Some people would replace heads and pistons to increase compression - building strokers basically. So, maybe it just depends the definition of 'bolt-on" I don't consider building a stroker "bolt on" even if there are plenty of bolts involved.
And of course there's the complexity issue. The more stuff you start throwing at some of these engines the more complex they can be (again, relatively speaking, of course).
So…. OP. If you want the most bang - pull your motor and go Bi-Turbo. Below that, I'd say go supercharger Much less hassle and fabrication. Other than that, you've got the old standbys of Cold Air Intakes, Throttle body spacers and "chips' (tuning). Personally I prefer a corn based chip to a potato based chip (with extra hot salsa) but that's just my preference. I don't think 'chipping' (tuning) a stock truck is gonna do a whole lot without being smart about the rest of the system - but it's your money. And I still find myself looking at those damn K&N filters at the local Pep Boys when I'm in there buying wax and what not.
Your ride will feel a little zippier, just like new tires are always grippier. Your ride might sound a little faster (Magnaflows!) and it might even be. But ultimately you have to ask yourself if the juice is worth the squeeze. Is $500 really worth 5HP? No one can answer that….
Ironically, there's an ad for "gforcechip' at the bottom of this thread advertising a $69 'chip' that will give you 60HP and 7 MPG. I call bullsh1t:
http://www.gfchips.com/toyota4runner.aspx
The number one speed increase for me is always a couple of beers and some good tunes. :beer:

olice:
Wait, I mean, a clean car. Yeah, clean cars definitely run better. Wash your car and enjoy.
BTW.. OP, nice 4Runner. I like the stance of those wheels. Look forward to seeing pics with the bumper!
Oh, almost forgot. Pacers rule!